1. Field of the Invention
The present invention relates to a process for production of iron oxyhydroxide particles, which can be used for magnetic recording media, lapping tapes, ink ribbons, cosmetics, coating paints and the like, and which are particularly suitable for use as precursors for magnetic particles in high-density, high-capacity magnetic recording tapes, and as a filler for support surface-smoothing coating materials.
2. Related Background Art
Demand continues to increase for magnetic recording tapes with larger volumes and higher densities, for which purpose such tapes are designed for smaller recording wavelengths and decreased bit lengths or track widths in order to increase recording density. Increased density, in other words, is achieved by improving the recording density and track density. Reducing the bit length and track width, however, decreases the number of magnetic bodies per bit and lowers the SN ratio, thus requiring the use of finer magnetic particles. Moreover, variation in the particle size results in non-uniform dispersion of the magnetic bodies, thus increasing noise. Consequently, the goal of increased density can only be achieved by forming fine particles of uniform particle size.
Wet synthesis methods are known for obtaining iron oxyhydroxide particles as precursors of needle-like magnetic particles. Such wet synthesis methods form iron oxyhydroxide particles by oxidation of ferrous hydroxide obtained by mixing and stirring a starting iron aqueous solution and a neutralizer. The iron oxyhydroxide particles, coated and/or doped with a sintering inhibitor, are then subjected to reduction treatment to obtain needle-like magnetic particles comprising Fe as a constituent element. In order to obtain needle-like magnetic particles with the required level of fineness it is important to control the oxidizing conditions in the ferrous hydroxide nucleation step. Rapid and homogeneous oxidation reaction between gas and liquid is important for controlling the oxidizing conditions. It is known that oxidation reaction is accelerated by controlling the oxidizing conditions, such as the gas/liquid mixing method and the oxygen component ratio. The oxygen component ratio is the volume ratio of oxygen per unit volume.
For control of oxidizing conditions such as the gas/liquid mixing method and oxygen component ratio, there have been hitherto proposed methods of, for example, carrying out the mixing by running gas through a porous plate with a diameter of 2-5 mm to generate fine bubbles, methods of controlling the oxidation rate with an oxygen partial pressure of at least 0.2 atm for the oxidizing gas (for example, Japanese Unexamined Patent Publication HEI No. 3-228829), and methods of dividing the oxidizing step into two stages and further varying the oxidation rate (for example, Japanese Unexamined Patent Publication HEI No. 10-182162).
However, Japanese Unexamined Patent Publication HEI No. 3-228829 cited above teaches that it is difficult to stably obtain fine goethite with an agitating tank reactor or bubble tower reactor, and its production process does not employ an agitating tank reactor. Consequently, since oxidizing gas with an oxygen partial pressure of 0.2 atm or greater is blown in without agitation, the obtained iron oxyhydroxide particles can potentially vary in their particle sizes. In Japanese Unexamined Patent Publication HEI No. 10-182162, the oxidation rate is increased to a prescribed oxidation rate during the step of oxidizing the ferrous hydroxide, and therefore considerable variation can potentially result in the particle sizes of the iron oxyhydroxide particles. Moreover, the particle length (long axis length) of the obtained goethite is approximately 0.05-0.25 μm, which is not a sufficiently micronized level.